In the field of high pressure sodium lamps, it is a common practice to provide an arc tube fill which includes a large amount of sodium and mercury in order to compensate for the undesired sodium losses encountered. These excess amounts of sodium and mercury result in an amalgam at the coolest points of the arc tube which is normally adjacent the electrodes at the ends of the arc tube. As a result, undesired variations in source voltage, color rendition and numerous other characteristics are encountered.
In an effort to eliminate or at least reduce such undesired effects, it has long been known that a lamp wherein the amount of sodium and mercury employed is only that which will become totally vaporized would provide the desired result. In other words, a high pressure sodium lamp of the unsaturated vapor type wherein sodium and mercury are introduced in only such an amount as to become totally vaporized is a highly desirable structure insofar as efficiency, cost of manufacture and enhanced lighting capability are concerned.
However, one of the major problems encountered in the fabrication of unsaturated vapor high pressure sodium lamps is the introduction therein of the proper amounts of sodium and mercury. Since the sodium content is of a relatively small amount and sodium is such a chemically active material, it has been found most difficult to dose or introduce the proper amount thereof into the arc tube of an unsaturated vapor high pressure sodium lamp.
One known suggestion for dosing an arc tube for a high pressure sodium lamp with the prop of sodium and mercury is set forth in U.S. Pat. No. 4,156,550, issued to Furukubo et al on May 29, 1979. Therein, sodium azide (NaN3) was dissolved in a solvent, placed in a container and the solvent evaporated. Also, a mercury dispenser in the form of an Al-Zr-Ti-Hg alloy was placed in the container. Thereafter, the container was positioned within one exhaust pipe affixed to the arc tube, and this one exhaust pipe was closed or pinched off. Another exhaust pipe or tube was affixed to the other end of the arc tube and to an exhaust system. The exhaust tube having the container therein was heated to decompose the sodium and mercury-containing compounds and provide the desired sodium and mercury within the arc tube. Also, the arc tube was evacuated and re-filled with a starting gas.
Although the above-described technique may or may not be employed in an unsaturated vapor high pressure sodium lamp fabrication process, it is submitted that the process leaves something to be desired. More specifically, the process appears to be relatively expensive of components, procedural steps and apparatus. For example, the suggested exhaust tubes of niobium are relatively expensive and not readily available in ordinary high pressure sodium lamp manufacturing facilities.